As being self-luminescent, EL devices have high visibility. In addition, they have high impact resistance as being completely solid devices. Therefore, the use of EL devices in various displays as light emitters is being widely noticed.
EL devices are grouped into inorganic EL devices in which are used inorganic compounds as light-emitting materials, and organic EL devices in which are used light-emitting organic compounds. Of those, organic EL devices have been being much studied and expected as light emitters in the coming generations, since they require a greatly reduced level of voltage, they can be easily small-sized, they consume small electric power, they can emit light in a mode of plane emission, and they can easily emit three primary colors.
Known are various structures of organic EL devices having a basic constitution of positive electrode/organic light-emitting layer/negative electrode and optionally provided with a hole injection and transportation layer and an electron injection layer, such as positive electrode/hole transportation layer/organic light-emitting layer/negative electrode, and positive electrode/hole transportation layer/organic light-emitting layer/electron injection layer/negative electrode, etc.
For such organic EL devices, for example, metal complexes of phenolato-substituted 8-hydroxyquinolines have been disclosed as blue-emitting materials (see Japanese Patent Application Laid-Open No. 5-198378). However, these are problematic in that their luminous efficiency is low to be at most 0.2 lumen(lm)/W or so. The reason is because the fluorescence quantum efficiency of the host compounds is low to be at most 0.07 or so. Doping of fluorescent substances into the host compounds is effective in prolonging the life of the doped host compounds, but is not in improving their luminous efficiency.
As luminous materials (host materials) for organic EL devices capable of emitting blue at high efficiency, distyrylarylene compounds have been disclosed (see Japanese Patent Application Laid-Open No. 2-247278). In addition, it has been disclosed that the doping of organic host compounds with fluorescent substances produces the improvement in the luminous efficiency of the doped organic host compounds while prolonging their life (see International Patent Application Laid-Open No. 94/06157).
Many of those blue-emitting compounds generally have a low glass transition temperature (Tg), since .pi.-electrons are distributed in narrow regions therein and since they have a low molecular weight. Therefore, organic EL devices comprising those compounds are problematic in that their thermal stability is poor. Organic EL devices for outdoor applications or applications in vehicles require high-temperature storage stability generally at 75.degree. C. or so. However, conventional organic EL devices are problematic in those applications in that, when they are kept at high temperatures of 75.degree. C. or so, the color to be emitted by them varies and their luminous efficiency is lowered. For these reasons, the applications of organic EL devices are inevitably limited.
Given that situation, various studies have been made in order to improve the thermal stability of organic EL devices. One example is to modify luminous materials to have dimer or oligomer structures, thereby making the materials have an elevated glass transition temperature. For this, referred to is Japanese Patent Application Laid-Open No. 8-12600, which discloses compounds (phenylanthracene derivatives) having a glass transition temperature of 181.degree. C. In this publication, they tried to improve the efficiency of their devices and to prolong the life thereof by mixing the hole transportation layer and the light-emitting layer. However, the devices disclosed have a luminous efficiency of at most 0.6 lm/W, which is lower than 1 lm/W, and the capacity of the devices is not satisfactory.
As has been mentioned hereinabove, no blue-emitting, organic EL devices having a long life, high efficiency and good thermal stability, which are all indispensable in their practical use, are unknown.